The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for under the terms of Department of Energy contract number DE-AC05-96OR22464.
Over the last twenty years, the development of continuous fiber-reinforced ceramics (CFCCs) has been driven, to a large extent, by the promise of substantial economic and environmental benefits if these materials are used in advanced industrial applications. M. A. Karnitz, D. F. Craig and S. L. Richlen, "Continuous Fiber Ceramic Composites Program," Am. Ceram. Soc. Bull., 70, 3 (1991) pp. 430-5.
Except for a few applications at "low temperature" when ceramics may offer superior chemical, wear, or electrical properties over conventional materials, the main attraction for using ceramic materials in industrial applications is their potential to retain strength at elevated temperatures in aggressive environments.
For example, the successful development and operation of advanced coal-fueled gas turbine technologies depends on the ability of filtration systems to clean fuel or combustion gas streams of particulates prior to these gases being passed through a turbine. R. R. Judkins, D. P. Stinton, R. G. Smith et al., "Development of Ceramic Composite Hot-Gas Filters," Journal of Engineering for Gas Turbines and Power, 118 (1996) pp. 495-499. Currently, CFCC filters are undergoing testing and demonstration throughout the world for application in pressurized fluidized-bed combustion (PFBC) and integrated gasification combined cycle (IGCC) power plants. R. R. Judkins, D. P. Stinton, R. G. Smith et al., "Development of Ceramic Composite Hot-Gas Filters," Journal of Engineering for Gas Turbines and Power, 118 (1996) pp. 495-499. In such applications, the filters are repeatedly cleaned by pulses of gases that impose thermal shocks on the filters, because the cleaning gas is typically at a much lower temperature than the gas being filtered. Therefore, CFCCs are preferred over monolithic ceramics because the former are tougher, have the potential of exhibiting a graceful, non-catastrophic mode of failure, and offer superior resistance to thermal shock.
The characteristics of the reinforcement/matrix interface in CFCCs are to a large extent, responsible for the adequate operation of the micromechanical mechanisms responsible for the tough behavior of these materials. These mechanisms are based on the debonding of the reinforcing fibers at the tip of an advancing matrix crack, on the bridging of the matrix crack by the reinforcing fibers, and on the pullout of the fibers in the wake of the matrix crack.
The best room temperature tensile behavior and toughness (as measured by the area under the tensile stress-strain curve) of CFCCs have been obtained with CFCCs reinforced with either carbon or SiC-based fibers, wherein the fibers are coated with a thin (0.1-0.5 .mu.m) compliant layer of carbon or boron nitride known as an interfacial coating. However, the use of CFCCs with carbon or boron nitride interfacial coatings is limited because in many applications, such as in PFBCs, the CFCCs must remain stable in an oxidizing environment at elevated temperatures, and neither carbon nor boron nitride meet this requirement.
Although it is widely accepted that non-oxide CFCCs will not be used in designs that would subject the component to stresses larger than the so-called matrix cracking stress, accidental stress excursions beyond this stress invariably will occur and result in matrix cracking. Matrix cracks serve as avenues for the ingress of the environment to the interior of the composite, thereby leading to oxidation of the fiber coating, and in the case of non-oxide fibers, to oxidation of the fibers and ultimately failure of the composite. E. Lara-Curzio, "Stress-Rupture of Nicalon.TM./SiC Continuous Fiber Ceramic Composite in Air at 950.degree. C.," J. Am. Ceram. Soc., 80[12](1997) pp. 3268-3272.
As a result of these limitations, substantial efforts have been expended in recent years for functional interfacial coatings that would protect the fibers during composite processing, be thermochemically stable with the fibers, the matrix, and the service environment, and be capable of promoting the deflection of matrix cracks allowing for fiber debonding and sliding. Furthermore, these conditions should be fulfilled over the expected service life of the component, which in the case of filters, for example, is measured in tens of thousands of hours.
Prior art attempts to provide oxidation resistant fiber coatings include doping carbon and BN with various element to improve their oxidation resistance, R. A. Lowden, O. J. Schwartz, and K. L. More, "Improved Fiber Coatings for Nicalon.TM./SiC Composites," Ceram. Eng. Proc., 14, [7-8] (1993) pp. 375-384; S. Jacques, A. Guette, F. Langlais and R. Naslain, "Preparation and Characterization of SiC/SiC Microcomposites with Composition Graded C(B) Interphase," Key Eng. Mat, 127, Part 1 & 2 (1997) pp. 543-50; A. W. Moore, H. Sayir, S. C. Farmer, and G. N. Morscher, "Improved Interface Coatings for SiC Fibers in Ceramic Composites," Ceram. Eng. Sci. Proc., 16 [4] (1995) pp. 409-416, the use of multilayered fiber coatings of carbon and SiC H. W. Carpenter, and J. W. Buhlen, "Fiber Coatings for Ceramic Matrix Composites," Ceram. Eng. Sci. Proc., 13 [7-8] (1992) pp. 238-256; C. Droillard, Ph.D. Thesis, University of Bordeaux, France (1993); R. Naslain, "The Concept of Layered Interphases in SiC/SiC," Ceram. Trans. 58 (1995) pp. 23-39, easy-cleaving oxides (e.g. silicates that are high-temperature analogues of mica, .beta.-alumina, magnetoplumbites), H. Beall, K. Chyung, S. B. Dawes, K. P. Gadkareer and S. N. Hoda, "Fiber-reinforced Composite Comprising Mica Matrix or Interlayer" U.S. Pat. No. 4,948,758, Aug. 14, 1990; and M. K. Cinibulk, "Magnetoplumbite Compounds as Fiber Coating for Oxide-Oxide Composites," Ceram. Eng. Sci. Proc., 15[5] (1994) pp. 721-728, low strength porous and pseudo-porous fiber coatings, H. W. Carpenter, and J. W. Buhlen, "Fiber Coatings for Ceramic Matrix Composites," Ceram. Eng. Sci. Proc., 13[7-8] (1992) pp. 238-256; H. W. Carpenter, J. W. Bohlen, and W. S. Steffier, "Weak Frangible Fiber Coating with Unfilled Pores for Toughening Ceramic-Matrix Composites," U.S. Pat. No. 5,221,578, Jun. 22, 1993; R. S. Hay, "The Use of Solid-State Reactions with Volume Loss to Engineer Stress and Porosity into the Matrix-Fiber Interface of a Ceramic Composite," Acta. Metall. Mater., 43, 9 (1995) pp. 3333-3347; L. U. J. T. Ogbuji, "A Porous Oxidation-Resistant Fiber Coating for CMC Interphase," Ceram. Eng. Sci. Proc., 14 [4] (1995) pp. 497-505, fugitive layers that disappear during processing leaving a physical gap between within the interfacial coating T. Mah, K. Keller, T. A. Parthasarathy, and J. Guth, "Fugitive Interfacial Coating in Oxide/Oxide Composites: A Viability Study," Ceram. Eng. Sci. Proc., 12[9-10] (1991) pp. 1802-1815, and isotropic oxide interfacial coatings such as rare-earth orthophosphates, zirconia, scheelite, alumina, mullite, and tin dioxide, to list just a few, D. B. Marshall, J. B. Davis, P. E. D. Morgan, and J. R. Porter, "Interface Materials for Damage-Tolerant Oxide Composites," Key Eng. Mat, 127, Part 1 & 2 (1997) pp. 27-36; G. Cain, R. L. Cain, A Tye, P. Rian, M. H. Lewis and J. Gent, "Structure and Stability of Synthetic Interphases in CMCs," Key Eng. Mat, 127, Part 1 & 2 (1997) pp. 37-50; W. Y. Lee, E. Lara-Curzio and K. L. More, "Multilayered Multifunctional Oxide Coating Concept for Satisfying Complex Interface Materials Criteria in Ceramic Matrix Composites" J. Am. Ceram. Soc., 81[4] pp. 600-604 (1998); G. Razzell, Zirconia Interface Layers Applied to Single Crystal Alumina Fibers by PVD and CVD," Key Eng. Mat, 127, Part 1 & 2 (1997) pp. 551-8; B. Davis, J. P. A. Lofvander, A. G. Evans, E. Bischoff, and M. L. Emiliani, "Fiber Coating Concepts for Brittle Matrix Composites," J. Am. Ceram. Soc., 76[5] (1993) pp. 1249-1257; R. W. Goettler, S. Sambasivan, and V. Dravid, "Isotropic Complex Oxides as Fiber Coatings for Oxide-Oxide CFCC," Ceram. Eng. Sci. Proc., 18[3] (1997) pp. 279-286; S. Sambasivan, "Rb-Stabilized .beta.-Al.sub.2 O.sub.3 as an Interphase Coating for CMCs," Ceram. Eng. Sci. Proc.,17[4] (1996) pp. 250-257; and, K. K. Chawla, M. K. Ferber and Z. R. Xu, "Interface Engineering in Alumina-Glass Composite," Mat. Sci. Eng. A., 162, (1993) pp. 35-44.
Still other approaches that have been used to extend the service life of CFCCs consist in using low-viscosity outer protective coatings or matrix additives to heal matrix cracks, and while some of these concepts might alleviate the lack of environmental stability of the fiber/matrix interface, they do not altogether solve it. Furthermore, matrix crack healing may be undesirable in filtration applications since it would be detrimental for the permeability characteristics of the filter material.
In view of the above, a need has existed in the art for an interfacial coating that will provide protection from oxidation in the occurrance of matrix cracking.